Temperature-compensated rod resonator

ABSTRACT

A temperature-compensated rod resonator, comprising a housing ( 10 ) having electrically conducting walls, including at least one electrically conductive resonator rod ( 14 ) extending from a bottom wall ( 11 ) towards a top wall ( 13 ), a temperature-compensating plate ( 20 ) located adjacent to said top wall ( 13 ) and coupling means ( 150, 151 ) for transferring electromagnetic energy to and from the resonator. The plate ( 20 ) is adapted to change its geometrical configuration in response to temperature variations. The temperature-compensating plate is a bimetallic plate ( 20 ) having a larger diameter than the resonator rod ( 14 ). A central portion ( 21 ) of said bimetallic plate ( 20 ) is secured to the upper end of the resonator rod ( 14 ), whereby the bimetallic plate, in conjunction with the adjacent top wall ( 13 ) defines a capacitance, which has a dominating influence on the resonance frequency. A pheripheral portion ( 22 ) of the bimetallic plate ( 20 ) is permitted to be freely deflected in response to the temperature variations, whereby the resonance frequency is changed so as to counteract temperature-induced dimensional changes of the housing ( 10 ) and the resonator rod ( 14 ).

This is a nationalization of PCT/SE00/00787, filed Apr. 26, 2000 andpublished in English.

FIELD OF THE INVENTION

The present invention relates to a temperature-compensated rodresonator, a filter including such a rod resonator, and a bimetallicplate for use in such a rod resonator. More particularly, the inventionconcerns a rod resonator comprising:

a housing having electrically conducting walls, including side walls, abottom wall and a top wall,

at least one electrically conductive resonator rod extending from saidbottom wall towards said top wall, an upper end portion of said rodbeing located at a predetermined distance from said top wall, so as todefine a resonance frequency,

a temperature-compensating plate located adjacent to said top wall andbeing adapted to change its geometrical configuration in response totemperature variations, and

coupling means for transferring electromagnetic energy to and from theresonator.

Such rod resonators are especially suitable as structural parts offilters in radio devices.

BACKGROUND OF THE INVENTION-PRIOR ART

There are resonators and filters of many different kinds, e.g., cavityresonators, coaxial resonators with a central rod (for example of thekind specified above), and dielectric filters. In all these kinds ofresonators attempts have been made to compensate for dimensional changescaused by temperature variations so as to keep the resonance frequencysubstantially constant.

A classical method is to combine various materials, having differentcoefficients of thermal expansion, in various portions of the resonator.Another way is to make use of bimetallic elements to achieve the desiredtemperature-compensation.

In a cavity resonator disclosed in U.S. Pat. No. 3,414,847 (Johnson),one of the walls defining a box-like cavity, or at least a part of sucha wall, is formed by a bimetallic disc which is movable in its entiretyin relation to the other walls of the cavity, primarily to enable tuningof the resonator. The disc is mounted on an axially movable plug orshaft, whereby the resonator can be tuned to a desired resonancefrequency. The bimetallic disc will change its geometrical shape whenthe temperature varies, and the structure aims at compensating thetemperature-induced dimensional changes by such a change of the shape ofthe disc. However, since the resonant frequency depends on the totalheight or length of the cavity, and the distance between the disc andthe opposite wall of the cavity is relatively large, the compensatingeffect will vary with the particular position of the disc obtained whentuning the resonator. Therefore, it is difficult to achieve an exacttemperature-compensation. Moreover, the overall dimensions of a cavityresonator of this kind are relatively large, at least in the frequencyrange of about 1-2 GHz.

A similar device is described in SU-836-711 (Savshinskii), where thecompensating element is an elastic, cupola-shaped plate, which isperipherally secured in a metallic holder having a different coefficientof thermal expansion than that of the plate. The flexure of the plate,which is temperature-dependent, will determine the effective length ofthe cavity. However, the same difficulties appear as in the previousexample of prior art.

Similarly,U.S. Pat. No. 3,740,677 (Motorola) discloses a cavityresonator, where a plunger on a shaft is displaceable by means of twobimetallic washers mounted on the shaft. The respective peripheral edgesof the washers are secured to opposite sides of the plunger, whereby theplunger will be displaced in its entirety when the washers change theirshape in response to temperature variations.

Furthermore, a dielectric resonator with a temperature-compensatingbimetallic plate is disclosed in JP-3-22602. Here, the plate is mountedon a tuning screw in opposite relation to a dielectric resonator bodyhaving substantially the same diameter as the plate. Of course, in sucha dielectric resonator, the major part of the electromagnetic energy isconfined within the dielectric or ceramic body. Therefore, the effect ofthe change of the geometrical configuration of the plate is marginal.Moreover, with a relatively large tuning range, it will be virtuallyimpossible to achieve the desired temperature-compensation so as tomaintain the resonance frequency at a substatially constant value.

Another example of prior art resonators with a temperature-compensatingplate is the coaxial resonator disclosed in U.S. Pat. No. 5,304,968(LK-Products OY), which is of the kind defined in the first paragraphabove. The centre part of the plate is spaced at a distance from the topwall of the resonator, and the plate has two opposite edge partsattached to the top wall. The coefficients of thermal expansion aredifferent for the top wall and the plate. Therefore, the plate willchange its configuration when the temperature varies, whereby thecapacitance between the top wall and the free end of the resonator rodwill be changed. However, because of the small area of the free end ofthe rod, it is difficult to achieve a well-defined capacitance and aprecise temperature-compensation.

SUMMARY OF THE INVENTION

Against this background, a main object of the present invention is toachieve an improved temperature-compensation of a resonator of the kinddefined in the first paragraph so as to keep the resonance frequency ata substantially constant value in spite of inevitable variations intemperature.

A further object is to enable the use of materials which are lesstemperature stable and to select suitable materials without therequirement of mixing materials having different coefficients of thermalexpansion.

A still further object is to permit tuning of the resonant frequencyindependently of the measures required for temperature-compensation.

Yet another object of the invention is to provide a resonator havingsmall dimensions and which is relatively easy to manufacture.

These objects are achieved for a resonator according to the invention,which has the following features:

The temperature-compensating plate is a bimetallic plate having a largerdiameter than the resonator rod. The central portion of the bimetallicplate is secured to the upper end of the resonator rod, whereby thebimetallic plate, in conjunction with the adjacent top wall, defines acapacitance, which has a dominating influence on the resonance frequencywhile providing a reduction of the geometrical length of the rodcompared to a rod without such a plate. Moreover, the peripheral portionof the bimetallic plate is permitted to be freely deflected in responseto temperature variations, whereby the capacitance between thebimetallic plate and the top wall is changed so as to counteracttemperature-induced dimensional changes of the housing and the resonatorrod.

Tests have shown that it is possible to achieve a very stable resonancefrequency with a rod resonator having such a structure. Because of therelatively large effective area of the bimetallic plate, the topcapacitance (between the plate and the top wall) can be maintained at ahigh value while keeping a certain minimum distance therebetween,whereby the tolerances of the structural elements (the top wall and theplate) can be held at reasonable levels which facilitate themanufacturing of the resonator.

Also, the power handling capability can be increased because of therelatively large gap between the upper end of the rod and the top wall.So, the risk of a corona breakdown will be lowered.

Basically, the bimetallic plate, at least the central portion thereof,will be stationary because its central portion is fixedly secured to thetop end portion of the fixed resonator rod. Even if tuning is carriedout, for example by means of a tuning element located at the adjacenttop wall, the bimetallic plate and the adjacent top wall are heldstationary in relation to each other. Thus, in the region where thetemperature compensation is performed, i.e. at the peripheral portion ofthe bimetallic plate, there will be no change as a consequence of thetuning process. Therefore, the temperature compensation will besubstantially uneffected by the tuning.

It has turned out that the manufacture of a rod resonator according tothe invention is relatively easy and inexpensive. The housing can bemade of aluminium in a moulding process, and the materials for otherparts of the resonator can be selected at will without considering thevarious coefficients of thermal expansion.

Moreover, thanks to the relatively short geometrical length of theresonator rod, the overall dimensions of the resonator, and any filtercontaining one or more such resonators, will be small. Of course, thisis a great advantage in many practical applications, such as radiodevices, for example in base stations for mobile telephone systems andthe like.

From a practical point of view, it may also be advantageous to useplastic materials, coated with an electrically conductive material, forthe housing and possibly also the resonator rod. Of course, the rod maybe made of a different material than the housing as long as the surfaceportion thereof is electrically conducting.

As indicated above, it is important that the bimetallic plate issecurely fastened to the top end portion of the resonator rod. This canbe accomplished in a practical manner by making the bimetallic plate inthe form of a ring member with a hole corresponding substantially to thecross-sectional shape of the resonator rod (at the upper end portionthereof—in principle, the resonator rod may have a cross-section whichis different at various longitudinal sections thereof). A preferred wayof securing the plate is to use a rivet connection. These and otherfurther features will appear from the appended claims.

The invention will be described more fully below with reference to thedrawings, which illustrate some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in a schematic sectional side view, a rod resonatoraccording to a first embodiment;

FIGS. 2 through 5 show, in partial views to a larger scale, variousmodifications of the connection between the rod and the bimetallic plateincluded in the rod resonator of FIG. 1;

FIG. 6 shows, likewise in a schematic sectional side view, a secondembodiment of the resonator including three rods.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The resonator illustrated in FIG. 1 comprises a cylindrical or box-likehousing 10 including a bottom wall 11, side walls 12 and a top wall 13,formed as lid, as well as a central resonator rod 14, normally having anelectrical length corresponding to a quarter of the wave-length (at thenormal operating resonant frequency). The walls 11-13 of the housing 10as well as the rod 14 can be made of an electrically conductingmaterial, e.g., a metallic material, such as Al. Alternatively, theseelements can be made of a plastic material coated with an electricallyconductive material at the inside, so that the cavity 15 formed withinthe housing 10 is defined by electrically conducting wall surfaces. Theresonator described so far is a coaxial resonator wherein anelectro-magnetic field can be excited at a resonant frequency byconnecting the resonator to input and output coupling means (not shownin FIG. 1), as is known per se. Thus, the resonator can be used as aband pass filter with a pass band centered around the resonantfrequency.

As is also known to those skilled in the art, there is a tuning assembly16 at the central portion of the top wall 13, including a tuning screw17 and a locking nut 18. Hereby, the resonant frequency can be tuned toa desired value within certain limits.

According to the invention, a bimetallic plate 20 is mounted at the topend portion of the resonator rod 14 in order to achievetemperature-compensation. The central portion 21 of the plate 20 issecurely fastened to the rod 14, whereas the peripheral portion 22thereof is permitted to deflect freely upwards and downwards in responseto temperature variations, as indicated by the dotted lines in FIG. 1.Hereby, the temperature-induced dimensional changes of the housing 10and the rod 14 will be counter-acted, so as to substantially reduce oreven eliminate an associated change of the resonant frequency, asdiscussed above. Also, the length of the rod 14 and the overalldimensions of the resonator are reduced thanks to the plate 20.

The outer diameter of the bimetallic plate 20 should be larger than thediameter of rod 14, preferably 1.5 to 4 times the latter diameter, inorder to obtain the advantageous effects mentioned above.

Preferably, as illustrated in FIGS. 2-5, the plate is a ring member 20′,20″ having a central hole 21′, which corresponds substantially to thecross-sectional shape of the resonator rod 14′, 14″. Advantageously, theupper end portion of the rod 14′ has a central recess or bore 23 whichcan partially accommodate the tuning screw 17, if necessary, withoutmaking contact with the latter.

The bore 23 will define an upper sleeve portion 24 of the rod 14′,provided with an abutment shoulder 25 formed by an external recess atthe top of the sleeve portion 24. Hereby, the bimetallic ring member 20′will be seated in a well-defined position. A secure fixation of the ringmember can be achieved by deforming the material of the sleeve portion24 against the inner edge of the hole 21′.

As an alternative, a separate bushing 26 can be inserted into thecentral recess 23. As shown in FIG. 3, a bottom flange or wall 27 issecured to the bottom of the recess 23 by means of a fastening screw 28.

The ring member 20′ may be bevelled at the upper edge of the hole 21′,asshown at 29 in FIG. 4, whereby the riveting of the sleeve 24 or bushing26 is facilitated and the secure holding of the ring member in a fixedposition is achieved.

A further modification of the connection between the rod 14″ and theplate 20″ is shown in FIG. 5, where a massive upper portion of the rod14″ is provided with an external circumferential groove 30 having acurved cross-section. The ring member 20″ has a rounded inner edge 31,which fits into the groove 30 and holds the ring member 20″ in positionwhile permitting a bending movement thereof.

FIG. 6 illustrates a second embodiment of a resonator according to theinvention, provided with three resonator rods 14 in a row in the samehousing 100. Each resonator rod 14 has a bimetallic plate 20, and atuning assembly 16 is disposed opposite to the respective resonator rod14 in the top wall 130. Input and output means 150, 151 are also shownin FIG. 6.

Thus, a filter may be composed of a number of resonator rods in ahousing. The various rods do not have to be located along a straightline but in any desired configuration. The configuration of the housing,defining a cavity with one or any desired number of resonator rods, mayalso be chosen at will.

The bimetallic plate does not have to be circular but may be square,polygonal or of any other form, preferably symmetrical with respect tothe axis of the resonator rod. As indicated above, the centre portion ofthe bimetallic plate may be massive or provided with a central hole.Also, the bimetallic plate does not have to be planar in its restposition but may be wholly or partially curved, e.g., as a bowl.

What is claimed is:
 1. A temperature-compensated rod resonator,comprising: a housing having electrically conducting walls, includingside walls, a bottom wall and a top wall, at least one electricallyconductive resonator rod extending from said bottom wall towards saidtop wall, an upper end portion of said rod being located at apredetermined distance from said top wall, to define a resonancefrequency, a temperature-compensating plate located adjacent to said topwall and being adapted to change geometrical configuration in responseto temperature variations, and coupling means for transferringelectro-magnetic energy to and from the resonator, saidtemperature-compensating plate is a bimetallic plate having a largerdiameter than said resonator rod, a central portion of said bimetallicplate being secured to said upper end of said resonator rod, thebimetallic plate, in conjunction with the adjacent top wall defines acapacitance, which has a dominating influence on said resonancefrequency, a peripheral portion of said bimetallic plate being permittedto be freely deflected in response to said temperature variations, andcapacitance between the bimetallic plate and said top wall is changed tocounteract temperature-induced dimensional changes of said housing andsaid resonator rod.
 2. The rod resonator is defined in claim 1, whereinthe diameter of said bimetallic plate is 1.5 to 4 times the diameter ofsaid resonator rod.
 3. The rod resonator as defined in claim 1, whereinsaid bimetallic plate is a ring member with a hole correspondingsubstantially to the cross-sectional shape of said resonator rod.
 4. Therod resonator as defined in claim 3, wherein a tuning member is disposedin said top wall opposite to said bimetallic ring member, and whereinsaid upper end portion of said resonator rod has a central recess with adiameter being substantially larger than the diameter of said tuningmember.
 5. The rod resonator as defined in claim 4, wherein thebimetallic ring member is mechanically secured to said upper end portionof said resonator rod by a sleeve portion extending axially through thehole of the bimetallic ring member.
 6. The rod resonator as defined inclaim e wherein said bimetallic ring member is secured to said resonatorrod by a rivet connection.
 7. The rod resonator as defined in claim 5,wherein an upper part of said resonator rod comprises a sleeve portion,the external circumferential surface of which is recessed so as to forman abutment shoulder for positioning said bimetallic ring member ontosaid resonator rod.
 8. The rod resonator as defined in claim 5, whereinsaid sleeve portion is a separate bushing having an upper flange andbeing secured at its lower end to the bottom portion of said centralrecess in the resonator rod.
 9. The rod resonator as defined in claim 8,wherein said bushing has a bottom flange with a hole for a screwfastener.
 10. The rod resonator as defined in claim 6, wherein saidbimetallic ring member has a beveled edge portion at the upper part ofsaid hole.
 11. The rod resonator as defined in claim 3, wherein the holeof said bimetallic ring member is seated in a circumferential groove ina cylindrical external surface of said resonator rod.
 12. The rodresonator as defined in claim 11, wherein a inner edge portion, whichdefines said hole of said bimetallic ring member, and saidcircumferential groove both have a curved cross-sectional shape.
 13. Afilter including at least one rod resonator as claimed in claim
 1. 14.The rod resonator as defined in claim 8, wherein said bushing has a wallwith a hole for a screw fastener.